In the process of biologically cleaning waste water, organic pollutants are converted into innocuous materials by micro organisms—referred to hereinafter as “biomass”—using oxygen. The waste water is cleaned by gassing a biomass-water mixture in a reaction vessel to which waste water and gas are continuously supplied and a comparable quantity of biomass-water mixture is extracted. The term “gas” is to be understood as meaning air, air enriched with oxygen or pure oxygen gas.
Organically charged waste water is produced in the domestic sphere as well as in many industrial processes. Methods for eliminating the dissolved organic compounds in an aerobic manner by means of micro organisms for the purposes of cleaning such waste water are known. These methods are usually carried out in flat aeration tanks. The disadvantages of such methods, such as the noxious smell produced in the surrounding environment due to the high quantities of exhaust air, the high noise level, the large amount of space required as well as the high investment and energy costs are well known.
A process of cleaning waste water in high cylindrical towers similar to a bubble column is also known (DE-Z “Chemie-Ingenieur Technnik” 54 (1982), No. 11, pages 939 to 952). However, because of the unfavourable hydrodynamic conditions and thus the relatively poor mass transfer properties of bubble columns, their space loading is comparatively low, this being the quantity of pollutants measured in CSB in relation to the volume of the reaction vessel and the day. It is approximately 1 kg CSB/m3d. The gassing process is effected exclusively at the base of the respectively utilised tower whereby the specific energy requirements for this method are relatively high.
A method is known from DE 41 05726 C1 wherein a reaction vessel is used which is referred to in the specialist field as a “loop-type bubble column reactor”. A loop-type bubble column is a cylindrical container arranged such that its axis is vertical and in the interior whereof there is placed a guide tube that is open at both ends and likewise arranged along its vertical axis. In the reaction vessel according to this publication, a two-component nozzle by means of which the waste water and gas are supplied is arranged above the guide tube. Here, the gas is supplied through a straight tube and a surrounding channel to a first mixing path of the two-component nozzle to which the waste water is also supplied via a ring channel. A second mixing path and a third mixing path adjoin the first mixing path in each case with a step or edge to edge. In addition, between the second and the third mixing path, there is arranged a ring channel having suction nozzles through which waste water and gas are sucked in, these being mixed in the third mixing path by the substrate located in the two-component nozzle.
In the method according to DE 198 42 332 A1, an upper plug-in tube and a lower plug-in tube separated by a partition plate are present in the reaction vessel which is likewise implemented as a loop-type bubble column reactor. The gas is supplied to the reaction vessel by a gassing unit which is arranged on the base of the reaction vessel. A nozzle consisting of two concentric tubes is placed in the vicinity of the partition plate and a liquid being delivered by a pump flows downwardly through said nozzle into the lower plug-in tube. Further liquid is thereby entrained by this one-component nozzle.
The reactor according to DE 40 12 300 A1 which, is likewise implemented as a loop-type bubble column reactor, comprises an insert in the form of a flow guide tube having a nozzle at the lower end thereof although the construction of the nozzle is not described in the publication. Gas is supplied to this nozzle through a first line and waste water is supplied thereto through a second line. In accordance with the drawings of this publication, the nozzle part serving for the supply of gas is longer than the nozzle part intended for the waste water.
The reactor according to DE 37 03 824 A1, which is also implemented as a loop-type bubble column reactor, has an insert in the form of a guide tube (with a deflector) into which a nozzle deeply projects. It has a central boring, a first annular gap for the supply of gas concentrically surrounding the boring and a second annular gap of the same axial length which concentrically surrounds the first annular gap. The waste water requiring cleaning is supplied through the central boring and the annular gap. The central boring is axially longer than the first annular gap serving for the supply of gas.
A higher conversion rate is achieved by the waste water treatment method of EP 0 130 499 B1 described hereinabove. This has a space loading of up to 70 kg CSB/m3d. In the case of the loop-type bubble column reactor according to this publication, waste water and gas are introduced into a guide tube through a two-component nozzle consisting of two shock-free, concentrically arranged tubes. The tube of the two-component nozzle serving for the supply of the gas is located within the outer tube serving for the supply of the waste water. It projects out from the outer tube. When this loop-type bubble column reactor is in operation, a current flow in the form of a loop develops around the guide tube thereby mixing the liquid and the gas. The advantages of such a loop-type current flow are a relatively homogeneous flow of the two phases and, associated therewith, adequate transfer of the oxygen that is needed for the purification of the waste water from the gas to the liquid. In this known method, the gas is introduced into the guide tube from above so that it goes through a complete loop at least once before it can exit from the reaction vessel. However, the liquid supplied to the guide tube must provide a large amount of energy in order for the supplied gas that has been separated into bubbles to be transported downwardly by the loop-type current flow against its “propensity to rise”.